Airbag restraint systems for passenger vehicles are known in the art. Such systems typically include an actuation circuit and a diagnostic circuit. The actuation circuit includes at least one inertia switch connected in series with a detonating device, e.g., a squib, and a source of electrical energy. The diagnostic circuit tests the operativeness of the actuation circuit and controls actuation of an indicator to inform the vehicle operator of a detected system error. Such diagnostic circuits typically monitor voltage values at various test points in the actuation circuit and compares the monitored voltage values against predetermined limits. When a monitored voltage value is outside of its predetermined limits, a system error has occurred.
Airbag restraint systems receive their operating power from the vehicle battery. When vehicle deceleration exceeds a value sufficient to close the inertia switches in the restraint system, e.g., during a vehicle crash, a squib is "fired" and the airbag is inflated. The battery provides the electrical energy to fire a squib. Typical airbag restraint systems include a storage capacitor that functions as either a main power source or as a back-up power source should the actuation circuit become disconnected from the vehicle battery during a crash.
The storage capacitor in such airbag restraint systems must have a sufficient capacitance value to insure that an adequate supply of electrical energy will be available to fire the squib. Some prior art diagnostic circuits monitor the static, steady-state voltage developed across the capacitor. An incorrect capacitance value, however, cannot be detected by simply monitoring the static steady-state voltage developed across the capacitor.
U.S Pat. No. 3,714,627 discloses a diagnostic circuit for an airbag restraint system that tests the operativeness of a storage capacitor. The voltage developed at a connection terminal of the storage capacitor is compared to the charge voltage developed across a test capacitor during initial energization of the restraint system. If the voltage value at the terminal of the storage capacitor is greater than the charge across the test capacitor during this initial energization period, such occurrence indicates that the storage capacitor is open circuited. An error indication is provided to the vehicle operator upon such occurrence.
Another known diagnostic circuit for an airbag restraint system is shown in FIG. 1. A squib 10 has one terminal connected to electrical ground through an 0.1 Ohm resistor 12. The other terminal of the squib 10 is connected to capacitors 14, 16. Capacitor 14 is connected to a source of electrical energy V(up) through a diode 18 and a current limiting resistor 20. Capacitor 16 is connected to the source of electrical energy V(up) through a diode 18 and a current limiting resistor 22. The capacitors 14, 16 charge through the squib 10. A voltage is developed across each capacitor 14, 16 substantially equal to V(up). The current limiting resistors 20, 22 prevent the squib 10 from "firing" during the charging of the capacitors 14, 16.
The capacitors 14, 16 are connected to one terminal of an inertia switch 24 through diodes 26, 28, respectively. The other terminal of inertia switch 24 is connected to electrical ground. A resistor 29 is connected in parallel with the inertia switch 24. When the inertia switch 24 closes, the capacitors 14, 16 discharge thereby pulling current through the squib 10 of sufficient magnitude and duration to "fire" the squib.
The capacitors 14, 16 are further connected to field-effect-transistors ("FETs") 30, 32 through resistors 34, 36, respectively. Each FET 30, 32 is controllably connected to a microcomputer 38. The junction of resistor 20 and capacitor 14 is connected to a voltage dividing network 40 including resistors 42, 44 connected in series to electrical ground. The junction of resistor 22 and capacitor 16 is connected to a voltage dividing network 46 including resistors 48, 50 connected in series to electrical ground.
The junction of resistors 42, 44 is connected to an analog-to-digital ("A/D") converter 52. The junction of resistors 48, 50 is connected to the A/D converter 52. The A/D converter is operatively connected to the microcomputer 38. The microcomputer 38 is connected to an indicator 54.
The circuit shown in FIG. 1 tests the operativeness of capacitors 14, 16, serially, i.e., separately. The microcomputer 38 partially discharges one of the capacitors being tested. The microcomputer monitors the voltage across the capacitor being tested through its associated resistor network 40, 46 and its associated connection with the A/D converter 52. If the monitored voltage across the capacitor being tested, which is now partially discharged, is not greater than a predetermined limit, e.g., as would occur when the capacitor is open circuited or is not a proper value, the indicator 54 is energized to warn the vehicle operator of the detected error.
The capacitor test in the system shown in FIG. 1 requires a separate switching FET for each capacitor and a separate voltage dividing network connected to the A/D converter for each capacitor. Also, each capacitor test is time consuming. Because each airbag restraint system must be fully tested during the manufacturing process, such a long period of time needed to complete a test sequence is not desirable. It is, therefore, desirable to decrease the time needed for completion of a test sequence to decrease manufacturing time of the system.
Another concern with airbag diagnostic test circuits is an ability to monitor the operativeness of the system's inertia switches. To accomplish this goal, each inertia switch in known airbag restraint systems includes an associated resistor connected in parallel therewith. Each inertia switch resistor is connected in series with other inertia switch resistors of the system and with the squib. The inertia switch resistors and the squib form a voltage dividing network. A monitoring circuit monitors the voltage at connection terminals of the inertia switches. Based upon the monitored voltage values, the monitoring circuit determines if an inertia switch is electrically short circuited or electrically open circuited. The value of inertia switch resistors must be sufficient to limit the steady state current flow through the squib to a value well below that required to "fire" the squib. It is, therefore, desirable to be able to accurately measure the impedance of each inertia switch resistor to determine if its value is within predetermined limits.
U.S. Pat. No. 4,835,513 to McCurdy et al., and assigned to the assignee of the present application, discloses a method and apparatus for accurately measuring the impedance of each inertia switch resistor and for determining the operativeness of the system's storage capacitor.